I remembered this when reading about dragon lairs in the current edition of D&D where they are prone to have a lot of minions -- kobolds or lizardmen or something -- about.
The dragon is wiser than the EOL of that list.
There are lots of forces out there. The whole point of low-level minions is that they can deal with them. Unless the EOL was carefully monitoring the fight to see how good the fighters are -- and how can you conquer the world if you monitor every two-bit bunch that comes along? -- sending more minions will tell whether they were worn down enough.
True, it requires a reliable and plentiful source of low-level minions. On the other hand, if you send your crack troops against every band that can fight a bunch of low-level minions, it won't be long before you discover that was the valiant distraction, barely able to manage the first encounter, but able to draw away your elite so that the elite heroes can barge in, unopposed, and face you yourself, thus defeating the entire purpose of minions. It may be wise to hint that this is not the first bunch of heroes who came along -- far from. Bones may be too blatant -- on the other hand, they might not.